High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
Both the VGT and EGR regulate gas flow through the exhaust manifold and their effect is, therefore, jointly dependent upon the conditions in the exhaust manifold. Excessive EGR rates displace the intake of fresh air and may lead to incomplete combustion of the injected fuel which, in turn, could cause visible levels of smoke to occur. In addition, in engines equipped with a VGT, the actual flow through the EGR valve can vary greatly, even for a fixed EGR valve opening, due to exhaust pressure fluctuations generated by opening or closing the inlet guide vanes to the VGT.
Accordingly, for optimum engine performance, it is important to have accurate knowledge of the VGT actuator position. Specifically, in a typical engine control system, the VGT is used to regulate the mass airflow (MAF) and the EGR is used to regulate the intake manifold pressure (MAP). Because the steady-state engine map with respect to the VGT position is not monotonic, however, a controller that uses VGT to regulate MAF tracking must account for the VGT position. This is demonstrated in FIG. 1.
FIG. 1 shows the steady-state values of compressor mass airflow rate (MAF) at an engine speed of 2000 rpm, and a fueling rate of 4.0 kg/hr. Each line on the graph represents a constant EGR valve position wherein 0.0 is fully closed and 1.0 is fully open, and a varying VGT actuator position wherein 0.0 is fully closed and 1.0 is fully open. As can be seen from FIG. 1, at point 100 (X.sub.egr =1.0, X.sub.vgt =0.2), opening the VGT increases MAF, while at points 102 (X.sub.egr =0.0, X.sub.vgt =0.2) and 104 (X.sub.egr =0.0, X.sub.vgt =0.8) opening the VGT decreases MAF.
Thus, knowledge of the VGT position is important for effective MAF control and, hence, turbo-lag reduction. Position sensors for electronic and pneumatic VGT actuators, however, are often undesirable due to packaging constraints and added expense to the overall engine control system.
In addition, for monitoring and controlling the state of exhaust treatment devices such as diesel particulate traps, lean NOx traps, and catalytic converters, measurements of the turbine mass flow rate, exhaust manifold temperature, and turbine back-pressure are necessary. All of these measurements can be provided by mass flow, temperature, or pressure sensors, respectively, however, such sensors add additional expense and may add complexity to the overall engine control strategy.